The present invention relates to a method for inhibiting the formation and deposition of silica and silicate compounds in water systems with certain polymers containing fully substituted amide functional groups.
In many parts of the world, including Western United States, Mexico, and Southeast Asia, the efficient use of water in cooling, boiler, geothermal, and reverse osmosis applications is limited by the solubility of silica. Water supplies in these areas can have silica levels of from 30 to 120 parts per million (ppm). Water treatment operations are limited because the solubility of silica at about 150 ppm can be exceeded when minerals are concentrated during processing. This can result in the precipitation and deposition of amorphous silica and silicates with consequential loss of equipment efficiency.
The accumulation of silica on internal surfaces of water treatment equipment, such as boilers, cooling, and purification systems reduces heat transfer and fluid flow through heat exchange tubes and membranes. Soluble silica is mainly composed of silicic acid and silicates, in proportions that depend on pH. Insoluble silica originates from base-catalyzed condensation polymerization of soluble silica to produce nominally SiO.sub.2. The primary insoluble particle is smaller than 0.1 .mu.m and comprises a core of SiO.sub.2 with a surface of silanol groups (Si-OH). The polymerization rate is also pH dependant, with a maximum at about 8.0 to 8.5. However, group II metals, especially Ca, Mg, and Fe are almost always present with silica, and they can influence the rate of scale development. With Mg, for example, highly insoluble "magnesium silicate", can form, especially at pH&gt;8. This scale actually comprises silica particles with magnesium polysilicates on the particle surfaces. The removal of silica scale once it is formed is very difficult and costly. With high silica water, therefore, cooling and reverse osmosis (RO) systems typically operate at low water-use efficiency to assure that the solubility of silica is not exceeded. Under these conditions, RO systems must limit their pure water recovery rate and cooling systems must limit water recycling. In both cases, water discharge volumes are large.
Various additives have been employed to inhibit silica deposition. For example, several compositions based upon acrylic acid copolymers have been taught, such as U.S. Pat. No. 4,711,725 to Amick et al., which teaches acrylic acid copolymerized with acrylamido alkyl or aryl sulfonate, and substituted acrylamide, or U.S. Pat. No. 5,510,159 to Amjad, which teaches acrylic acid copolymerized with dimethyldiallylammonium chloride and acrylamide. U.S. Pat. No. 4,328,106 to Harrar et al. teaches inhibiting silica scaling and precipitation by injecting low concentrations of cationic nitrogen containing compounds, such as polymeric amines, polymeric imines, and quaternary ammonium compounds. U.S. Pat. No. 4,584,104 to Dubin teaches inhibiting amorphous silica scale formation by treating industrial waters with a boron compound which dissolves in or hydrolyzes in the industrial waters to give the orthoborate ion. U.S. Pat. No. 5,271,847 teaches controlling the deposition of silica by the use of a water soluble graft copolymer of acrylic acid and a polyalkylene glycol ether. U.S. Pat. No. 5,271,862 to Freese teaches inhibiting the deposition of silica and silicate compounds by adding a composition consisting of a hydroxyphosphono-acetic acid and a copolymer of acrylic acid and allyl hydroxypropyl sulfonate ether.
One commercially available polymeric material, namely PowderGuard.RTM. RO antiscalant, a powder antiscalant available from Pacific Aquatech Inc., has a suggested use for controlling fouling caused by a variety of sources, including silica; colloids; iron and aluminum hydroxides; barium, strontium, and calcium sulfate; and calcium phosphate and carbonate. Although no compositional information is available concerning PowerGuard.RTM. RO antiscalant, it is believed to be or to contain poly(vinyl pyrrolidone).